(a) State the underlying principle of a potentiometer. Why is it necessary to
(i) use a long wire,
(ii) have uniform area of cross-section of the wire ?
(b) Obtain an expression for equivalent capacitance when three capacitors `C_(1), C_(2) "and" C_(3)` are connected in series.

Explanation

### Step-by-Step Solution:

**(a) Underlying Principle of a Potentiometer:**

1. **Definition**: A potentiometer is an instrument used to measure the potential difference (voltage) across two points in a circuit.

2. **Principle**: The underlying principle of a potentiometer is based on the fact that the potential drop across a uniform wire carrying a steady current is directly proportional to its length. This can be mathematically expressed as:
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